Electromagnetic clutches are well known in the prior art and are often used for controlling the transfer of power from an automobile engine to the refrigerant compressor of an automobile air conditioning system.
With reference to FIG. 1, a standard electromagnetic clutch will be described. FIG. 1 is a cross-sectional view showing an electromagnetic clutch mounted on a refrigerant compressor. The electromagnetic clutch has ring-shaped rotor 2 rotatably supported on tubular projecting portion 3a of compressor housing 3 through radial bearing 4. A fan belt (not shown) will impart a rotational force to rotor 2 when the automobile engine is running. An electromagnet 5 is disposed in hollow portion 6 which is formed in rotor 2 and fixed on the compressor housing 3 by means of bolts. Electromagnet 5 comprises annular U-shaped housing 5a and electromagnetic coil 5b contained within housing 5a.
Drive shaft 7 is rotatably supported in compressor housing 3 through a radial bearing (not shown) which is mounted in tubular projecting portion 3a. Hub 8 is secured on the outer end of drive shaft 7 which extends from tubular projecting portion 3a and is connected to ring-shaped armature plate 10 through a plurality of leaf springs 9. Armature plate 10 is thus supported by leaf springs 9 around hub 8 and frictional surface 10a faces frictional surface 2a of rotor 2. An axial gap is formed between armature plate 10 and surface 2a and a radial gap between armature plate 10 and hub 8.
When coil 5b of electromagnetic 5 is energized, magnetic flux is generated and passes through annular housing 5a, rotor 2, and armature plate 10. Armature plate 10 is attracted to surface 2a of rotor 2. Accordingly, the rotational force supplied by the fan belt to rotor 2 is transmitted to drive shaft 7 through armature plate 10. When coil 5b of electromagnet 5 is deenergized, the magnetic flux dissipates and frictional surface 10a of armature plate 10 disengages from surface 2a of rotor 2 by the recoil strength of leaf springs 9, and the rototational force of rotor 2 is no longer transmitted to drive shaft 7.
In the above-described electromagnetic clutch, the frictional surface 2a of rotor 2 and the frictional surface 10a of armature plate 10 have irregular concaves and projections. A magnified view of such a surface is shown in FIG. 2. When two such surfaces initially make contact as described above, the actual contact area is below 20 percent of the entire surface.
Referring to FIG. 3, when the frictional surfaces of rotor 2 and armature plate 10 are initially engaged with one another, wedge-shaped projections 2b of rotor 2 and wedge-shaped projections 10b of armature plate 10 meet and a large frictional force is produced therebetween. This force generates shears on the top of the projections. Accordingly, after projections 10b and 2b are engaged with each other two or three times, the tops of the projections are shaved off, and the torque transferred to drive shaft 7 is decreased.
The contact area between the rotor and the armature plate will gradually increase due to the abrasion caused by continuous engagement, thus the torque transfer of electromagnetic clutch 1 will eventually increase and become three or four times the required value. However, before the clutch has been engaged a certain number of times, e.g., 50 times, the torque transfer of the electromagnetic clutch has not increased to the desired level.
To obtain a larger torque transfer when the clutch is initially operated, the area of contact between the two frictional surfaces can be enlarged, or the magnetic force can be increased by enlarging the sectional area of coil 5b. However, these methods of obtaining a larger torque transfer cause an increase in the size of the electromagnetic clutch and an increase in its weight.
Since both frictional surfaces have approximately the same hardness, the contact between both in the initial stages of operation will produce a deposit of high hardness between the surfaces, and the frictional surfaces will become rough.